The present invention relates to a substrate assembly system, using a meltable material.
Substrates may be assembled by means of meltable material by placing the substrates to be assembled into contact, heating a meltable material previously positioned in mutual bonding areas of the substrates and leaving said material to cool to solder the bonding areas together.
The invention may be applied to the production of electronic devices, particularly ultrahigh frequency circuits.
In ultrahigh frequency circuits, the invention may be implemented to attach a cover to the components. Such a cover is particularly intended to favour signal propagation and form a shield against parasitic electromagnetic radiation.
More generally, the invention may also be applied to the production of devices including microelectronic and/or micromechanical components requiring a shielding or protective part.
For example, the invention may be used in the production of accelerometers including an electronic measurement circuit and a seismic feeder, i.e. a micromechanical component liable to move under the effect of acceleration. In this example, a protective cover may be used particularly to prevent excessive movement of the feeder.
The most frequently used technique for assembling two substrates is the xe2x80x9cflip chipxe2x80x9d technique. This technique essentially comprises the following steps:
deposition of assembly beads or wafers made of a meltable material on bonding areas of the substrates to be assembled,
assembly of the substrates such that the pairs of bonding areas of each of the substrates respectively are placed opposite each other, at least one of the bonding areas of each pair being equipped with meltable material,
heating of substrates to bring the two parts to be assembled, placed opposite each other, to a temperature slightly above the melting point of the bead material, so as to carry out soldering, joining the bonding areas located respectively on each of the substrates together.
The role of the interconnection beads made of meltable material is essentially to ensure the cohesion of the structure formed by the assembled substrates. Another of the beads"" roles, particularly for assemblies including electronic circuits, is to supply a path for electrical contact between the assembled parts.
Finally, the beads may be used beneficially to adjust and maintain a determined spacing between the assembled substrates, in particular between a substrate comprising a component and a substrate forming a cover.
The last function may be essential, particularly in the formation of accelerometers wherein the cover is
used to adjust the travel or the maximum movement of a seismic feeder. The seismic feeder is in fact a fragile mechanical device and the cover is used to prevent excessive movement or deformation of the feeder liable to cause its rupture.
Although the assembly technique using beads made of meltable material is practical to use, its precision remains insufficient for adjusting a spacing between substrates which is less than or equal to 15 xcexcm.
To ensure precise and uniform spacing less than 15 xcexcm, it is essential to use special production and testing techniques relating to the level of the meltable material beads.
In particular, the meltable material of the beads or wafers may be formed electrolytically by dispensing or any other deposition technique. It is particularly formed into bonding areas comprising areas of material that can be wetted with the meltable material, also referred to as xe2x80x9cinterconnection blocksxe2x80x9d.
Special techniques to produce meltable material beads increase the number of steps in the process, particularly to prepare deposition or growth masks, and induce a higher assembly cost.
The state of the art is also illustrated in the following documents relating to component assembly:
D1: EP-A-0 558 325 (HUGHES AIRCRAFT COPANY) Feb. 25th 1993 (Feb. 25, 1993)
D2: DE 33 28 975-A-(ISRONICS INC Mar. 1st 1984 (Mar. 1, 1984)
D3: EP-A-0 616 238 (TEXAS INSTRUMENTS INC) Sep. 21st 1994 (Sep. 21, 1994)
D4: EP-A-0 809 289 (HARRIS CORP) Nov. 26th 1997 (Nov. 26, 1997)
D5: PATENT ABSTRACTS OF JAPAN, vol. 008, no. 099 (E-243) May 10th 1984 (May 10, 1984) and JP 59 01727-A-(NIPPON DENKI KK), Jan. 28th 1984 (Jan. 28, 1984).
The aim of the present invention is to propose a substrate and, more generally, a substrate assembly system without the limitations described above.
One of its specific aims is to propose a substrate and an assembly system not requiring additional processing operations.
Another aim is to propose an assembly system that is inexpensive to produce and implement.
A further aim is to propose an assembly system and method making it possible to adjust, simply and with great precision, the spacing between substrates, particularly spacing less than 15 xcexcm.
To achieve these aims, the invention more specifically relates to a system comprising a first substrate with at least one bonding area, liable to be assembled with a second substrate, the bonding area comprising an area made of a material that can be wetted with a meltable material. In addition, according to the invention, the bonding area comprises at least one cavity to receive meltable material.
According to the present invention, the term substrate refers to any part liable to be assembled with a meltable material. Therefore, a substrate may be, for example, a cover, a surface comprising mechanical, optical and/or electronic components, individually or in a circuit. The xe2x80x9csubstratexe2x80x9d may also consist of a micromechanical part, intended to be assembled with other parts to form a structure.
The present description of the invention relates to a simplified case in which the system of substrates to be assembled only comprises two substrates. It is understood that the invention applies to any system comprising a number of substrates greater than or equal to two.
The substrate bonding areas, particularly the areas of wettable material, are intended to receive the meltable material.
With the invention, the quantity of meltable material required to produce an assembly with a given spacing may be determined with a lower precision. Indeed, any excess meltable material may be collected in the cavities of the bonding areas. By setting their volume, the cavities also make it possible to adjust the level of meltable material assembling the two substrates.
In addition, since the meltable material can be collected in the cavities, it is possible, with good precision, to obtain spacing levels of the order of one micrometer between the substrates.
The walls of the cavities may be made of a material that can be wetted with the meltable material. When they are made of a wettable material, the walls of the cavity also contribute to the bonding, i.e. the cohesion of the structures.
Conversely, when they are made of a non-wettable material, the cavities receive any excess meltable material which is not fixed in the cavity. The non-wettable nature of the cavity walls may also be used beneficially to delimit the extension of the surface on which the solder is produced, i.e., definitively, to delimit the extension of the bonding area.
The bonding area may be delimited particularly by one or more cavities surrounding it.
For a given quantity of meltable material, the hybridisation level, i.e. the spacing level between the assembled substrates is essentially governed by the extension of the bonding area.
In this way, when the cavities are used to delimit the bonding area, the distance provided between the cavities makes it possible to adjust the hybridisation level.
In a specific case in which the volume of meltable material placed in a bonding area is less than the overall volume of the cavities associated with said bonding area, the hybridisation level is set automatically to a minimum value of the order of one micron.
Otherwise, the level is greater, and may be adjusted to a required value. If the surface area of the bonding area is known and a required assembly height is defined, it is possible to determine the volume of meltable material required.
This gives: Vf=H*S+Vc 
In these expression, Vf corresponds to the volume of meltable material, H to the assembly level, S to the surface area of the bonding area and Vc to the volume of the cavities.
Therefore, the equation giving the assembly height may be expressed as follows:   H  =                    V        f            -              V        c              S  
For example, for a bonding area of 2xc3x975 mm2 surrounded by four cavities: 2 cavities of 0.1xc3x975 mm2, 2 cavities of 0.1xc3x972 mm2, for a volume of meltable material of 20.8.10xe2x88x922 mm3, the assembly height will be approximately 10 xcexcm.
The invention also relates to an assembly system comprising a first substrate and at least a second substrate, the first and second substrates comprising bonding areas. The bonding areas of the first and second substrates are conjugated in pairs respectively such that a first bonding area of a pair is positioned opposite a second conjugated bonding area of the pair, when the first and second substrates are assembled. In addition, the bonding areas each comprise at least one area of material that can be wetted with a meltable material, and at least one of the bonding areas of a pair comprises a boss of meltable material in contact with the area of meltable material. According to the invention, at least one of the bonding areas of each pair comprises at least one cavity to receive meltable material.
The term boss refers to any agglomeration of meltable material liable to contribute to the assembly (or hybridisation) of substrates. Bosses may particularly come in the form of beads, wafers or small columns.
In a specific embodiment of the system, only the bonding areas of one of the substrates is equipped with bosses of meltable material, but free of cavities. The second substrate, comprising the conjugated bonding areas is free of meltable material (before assembly) but comprises the cavities intended to collect the excess meltable material during assembly.
The invention also relates to a method to produce a system comprising a first and a second substrate as described. According to this method:
the first and second substrates are assembled such that the bonding areas of a pair are opposite each other and separated by at least one boss of meltable material respectively,
the meltable material is heated to a sufficient temperature to solder the meltable material respectively on the bonding areas opposite each other and to induce the flow of excess meltable material in at least one cavity of the bonding areas, and
the meltable material is allowed to cool so that it sets.
The present invention""s other characteristics and advantages will be seen more clearly upon reading the following description, with reference to the figures appended. This description is given solely as an illustration and is not restrictive.